1. Field of the Invention
The invention is directed to a device for conveying liquid, particularly to a medication dosing unit.
2. Description of the Prior Art
Medication dosing units generally include an intermittently energized pump and a filter arranged at the input side thereof for retaining air bubbles, the filter being dimensioned such that air bubbles having a larger diameter than a prescribed minimum diameter are prevented from freely passing the filter. Additionally, the maximum pressure drop of the liquid flow produced by the filter given a pump action (stroke) is lower than the pressure required for conveying an air bubble having a larger diameter than the prescribed, minimum diameter through the filter.
Such a device disclosed in U.S. Pat. No. 4,604,090 is a medication dosing unit implantable in a living organism in which an electromagnetically actuatable bellows pump driven by electrical pulses draws a quantity of a liquid medication, corresponding to the working volume of the pump, by suction from a medication reservoir at every pulse. The pump supplies the medication to a catheter that conveys the medication portions to a suitable location in the organism, where the medication is injected. The bellows pump provided in the known apparatus has a relatively large dead volume, so that the medication conveying capacity of the pump decreases with increasing air accumulation when air proceeds into the pump, and the liquid conveying ultimately completely ceases because the quantity of air collected in the pump is merely compressed upon every pump action. A certain quantity of gas is always dissolved in the liquid to be pumped, and air can proceed into the medication reservoir when filling the medication reservoir under normal conditions if complicated countermeasures such as degasification of the liquid or filling the medication reservoir in a vacuum are not undertaken. Therefore, a filter for retaining air bubbles precedes the pump in the known apparatus. The filter is essentially composed of a flexible filter disc in the form of a porous membrane or in the form of a mesh grid having a pore width of 0.22 through 5 .mu.m, or a mesh width of, for example, 15 .mu.m, so that only extremely small air bubbles having the same diameter or a smaller diameter can pass the filter unimpeded. Due to the extremely small pore or mesh width of the filter, however, even a slight liquid flow through the filter can cause such a large pressure drop across the filter that larger air bubbles are also forced through the filter. In the known apparatus, the filter disc is fashioned so thinly, and thus so flexibly, that the filter disc is deflected upon every pump action by the suction pressure produced by the pump and then slowly returns to its initial position. As a result, the liquid flow through, and thus the pressure drop across, the filter is limited.
Due to the extremely small pore or mesh width of the filter, there is the risk in the known apparatus that the pores or meshes are very, quickly plugged by foreign particles or precipitants of the medication (for example, insulin). As a result--dependent on the flexibility of the filter disc--either the pressure drop as a consequence of the liquid flow through the pore or meshes that are not yet plugged becomes so great that air bubbles are forced through the filter, or the filter no longer returns into its initial position. The flow rate of the pump is diminished under either circumstance. Enlarging the pore or mesh width is precluded, however, because this would lead to an increased air permeability of the filter which must absolutely be avoided because of the aforementioned, relatively large dead volume of the known pump.
When the filter in the known apparatus is partially covered by the accumulating air at its side facing away from the pump, then this leads to the same consequences as in the case of the above-described plugging of the filter pores or meshes. Moreover, the pores or meshes act on the air bubbles like potential troughs wherein the air bubbles adhere and plug the pores or meshes. When the known filter is ultimately completely plugged by air bubbles or is covered by a large air bubble, the liquid conveying entirely ceases.